PC-DOS 4.0

by Lloyd Borrett
Technical Cornucopia, January–February 1989

This latest version of DOS boasts support for hard disk
volumes that hold an entire gigabyte (1 billion bytes) of
data. It also recognises the Expanded Memory Standard (EMS)
and can use it for its own buffers and RAM disk.

In addition, a simplified user interface or shell lets
you move through all disks, directories, and files, and
perform most actions with pull down menus and cursor keys or
a mouse. A minor, but nonetheless interesting, feature in
this version is support for 43 or 50 line text with an EGA
or VGA card.

In September 1988, IBM quietly started shipping PC-DOS
4.0 with disks labeled 4.01. This maintenance release fixed
some of the bugs that had caused user complaints but left
many other problems unanswered.

DOS 4.0 and EMS memory

PC-DOS 4.0 is the first product from IBM that recognises
EMS memory. Unfortunately IBM have not fully conformed to
the EMS 3.2, EEMS, or EMS 4.0 standards. They have
implemented their own form of EMS memory that only works
with the IBM XMA memory card.

IBM PC-DOS 4.0's use of EMS memory makes assumptions that
are entirely contrary to the standard. Thus existing users
with EMS 3.2, EEMS, or EMS 4.0 conforming memory cards, can
not use the EMS features in IBM PC-DOS 4.0. Thus the only
safe way to run IBM PC-DOS 4.0 is to leave it all in
standard memory and ignore the expanded memory options.

Microsoft have announced that their MS-DOS 4.0 will work
with any EMS 4.0 compatible memory card.

Larger hard disk volumes

Previous PC-DOS versions restricted hard disk size to 32
MBytes, viewing volumes larger than that as multiple hard
drives. PC-DOS 4.0 is the first version of PC-DOS that
supports hard disk volumes larger than 32 MBytes.

DOS 4.0 makes no changes in sector size or in the file
allocation table (FAT); it continues to use 512-byte
sectors, a 12-bit FAT for smaller disks, and a 16-bit FAT
for everything else. For disk volumes up to 128 MBytes, DOS
4.0 uses allocation units of 2 KBytes. For larger disks, it
simply increases cluster sizes. This means that no DOS 4.0
disk volume may ever hold more than 65,520 files.

The waiting game

Many popular utility programs, especially disk optimizers
and other programs that access disks directly, are
incompatible with DOS 4.0. Some are completely incompatible
with DOS 4.0, while others only have difficulties if you
have a disk volume that is larger than 32 MBytes. So if you
switch to DOS 4.0, you should plan to upgrade many of the
utility programs you use.

There are some conflicts between DOS 4.0 and some memory
resident programs. Some will not pop up at all, while others
have screen display conflicts. Many of the problems seem to
be caused by specific hardware configurations as they will
work okay on some systems but not on others.

We can assume that all of the incompatibilities and bugs
in PC-DOS 4.0 will eventually be ironed out by updates from
software manufacturers and bug fixes from IBM. It's to be
hoped that by the time Microsoft ships MS-DOS 4.0 it will
already have many of the problems resolved.

If you need hard disk volumes larger than 32 MBytes, DOS
4.0 may be the best choice for you. And once MS-DOS 4.0 is
available, or IBM solves their 4.0 EMS incompatibility
problems, moving DOS storage areas to expanded memory will
help keep large programs from running out of space.

However, I'll stick to using DOS 3.30 until the DOS 4.0
versions of the utility programs I use are available. Unless
you really need the expanded capabilities, you may be better
off sticking with your present operating system until DOS
4.1 appears.